Tin-silver coatings

ABSTRACT

The present invention relates to improved coatings for electrical or electronic connectors such as contacts or terminals used in automotive applications. Coatings in accordance with the present invention preferably comprise binary tin-silver coatings consisting of more than 1.0 wt % to about 20 wt %, preferably from 2.0 wt % to 15 wt %, and most preferably from 3.0 wt % to 10 wt %, silver and the balance essentially tin. The coating is preferably applied by immersing the substrate material in a molten tin-silver bath.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application hereby claims the benefit of U.S. ProvisionalPatent Application Ser. No. 60/312,331, filed Aug. 14, 2001, entitledTIN-SILVER COATINGS.

BACKGROUND OF THE INVENTION

The present invention relates to tin-silver coatings to be applied tosubstrate materials used in electrical and electronic applications, suchas in automotive connectors, and to a method of applying a tin-silvercoating to the substrate materials.

Electrical contacts in automobiles are subjected to a variety of hazardsbecause of the elevated temperatures and environment in which they mustfunction. For example, automotive electrical contacts are routinelysubjected to vibrations and fretting corrosion caused by micromotion.Fretting corrosion is a detriment because it elevates contact resistanceat the contacts of the electrical surface. Additionally, electricalarcing can occur when two electrical contacts or terminals are matedtogether.

To deal with some of these problems, some automotive electrical contactshave been coated with gold. Gold is an advantageous material because itdoes not lead to the production of deleterious oxides. The cost of goldhowever is very high and unduly increases the cost of the automotiveelectrical contact.

To eliminate the expense of the gold, some electrical contacts have beencoated with pure tin. While economically beneficial, a pure tin coatingdoes not have a very long cycle life, normally about 170 cycles.

Thus, there remains a need for a coating which can be used in theformation of electrical contacts which is economically beneficial andwhich can withstand the stresses of the environment under whichautomotive electrical contacts must function.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved coating for electrical and electronic applications.

It is a further object of the present invention to provide an improvedcoating as above which has particular use in automotive applications andwhich has a long cycle life.

It is a further object of the present invention to provide an improvedcoating as above for an automotive electrical contact which iseconomically acceptable.

It is a further object of the present invention to provide an improvedmethod for forming a coating on a substrate material.

It is yet another object of the present invention to provide an improvedmethod for forming an electrical connector or contact.

The foregoing objects are attained by the tin-silver coatings and themethod of the present invention.

In accordance with the present invention, a tin-silver coating isapplied to a substrate material to be used for an electrical contact.The substrate material may be any suitable metal having a desiredelectrical conductivity. For example, the substrate material can becopper, a copper alloy, a carbon steel material, or an aluminum alloy.The tin-silver coating of the present invention in a preferredembodiment consists of from more than 1.0 wt % to about 20 wt %,preferably from 2.0 wt % to 15 wt %, and most preferably from 3.0 wt %to 10 wt %, silver and the balance essentially tin. As will be discussedhereinafter, the binary tin-silver coatings of the present invention areparticularly advantageous coatings. For example, the coatings avoid thecreation of oxides which are deleterious to the coating and whichincrease the electrical resistance properties of the coating.

While it is preferred that the coatings of the present invention bebinary tin-silver coatings, the coatings may also contain an effectiveamount up to about 5.0 wt % of at least one hardening element selectedfrom the group consisting of bismuth, silicon, copper, magnesium, iron,nickel, manganese, zinc, antimony, and mixtures thereof. The at leastone hardening element when present in the coating should not be presentin an amount which causes the production of oxides which increase theelectrical resistance of the coating.

One advantage to the tin-silver coatings of the present invention isthat they do not need to include elements such as deoxidizing agents.

The tin-silver coatings of the present invention are formed using amethod which broadly comprises the steps of providing a substratematerial to be coated, preparing a bath consisting of from more than 1.0wt % to about 20 wt % silver and the balance essentially tin, andimmersing the substrate material in the bath to form a coating layer onthe substrate material, which coating layer consists of more than 1.0 wt% to about 20 wt % silver and the balance essentially tin. In analternative embodiment of the method of the present invention, the bathand the coating may also contain at least one element selected from thegroup consisting of bismuth, silicon, copper, magnesium, iron, nickel,manganese, zinc, antimony, and mixtures thereof. When present, the atleast one element should be present in an amount which does not causethe generation of oxides which increase the resistance of the coating.

Other details of the tin-silver coatings of the present invention andthe method for forming them, as well as other objects and advantagesattendant thereto, are set forth in the following detailed descriptionand the accompanying drawing(s).

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE illustrates the result of a hardness test comparing thehardness of a pure tin coating as compared to tin-silver coatings inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

As previously mentioned, the present invention relates to theapplication of a tin-silver coating on a substrate material to be usedin electrical and electronic applications such as an electrical contactfor an automobile. The substrate material may be any suitableelectrically conductive material known in the art such as a ferrousbased material, e.g., carbon steel material, or a non-ferrous basedmaterial, e.g., pure copper, a copper based alloy, or an aluminum alloy.One substrate material which is typically used in automotiveapplications is a copper-tellurium alloy designated as CDA Alloy 14530which contains from 0.003 to 0.023 tellurium, from 0.003 to 0.023 tin,from 0.001 to 0.010 phosphorous and the balance copper.

In most electrical and electronic applications, it is highly desirableto have a coating layer on at least a portion of the substrate materialto help prevent surface oxidation which causes fretting corrosion andincreased electrical resistance. The coating must have sufficienthardness to withstand the forces applied in such applications during alarge number of cycles, and a resistance value which is not too great.It has been found that a particularly useful composition for a coatingto be applied to a substrate material for use in electrical andelectronic applications, such as automotive applications, is a binarytin-silver coating which consists of more than 1.0 wt % to about 20 wt%, preferably from 2.0 wt % to 15 wt %, most preferably from 3.0 wt % to10 wt %, silver and the balance essentially tin. Such coatings have amelting point greater than 225° C., which is advantageous.

If desired, at least one hardening element selected from the groupconsisting of bismuth, silicon, copper, magnesium, iron, nickel,manganese, zinc, antimony, and mixtures thereof may be present in analternative embodiment of the tin-silver coatings of the presentinvention to increase the hardness properties of the coatings. Whenpresent, the at least one hardening element may be present in aneffective amount up to about 5.0 wt % total and preferably in an amountfrom 0.1 wt % to about 5.0 wt % total. The particular hardening elementor elements selected should not be present in an amount which createsdeleterious oxides sufficient to increase the electrical resistanceproperties of the coatings. For example, one should avoid the use ofsignificant amounts of oxide producing elements such as magnesium andcopper in the coatings.

The tin-silver coatings of the present invention may be applied to asubstrate material using any suitable technique known in the art. It ispreferred however to apply the tin-silver coating to the substratematerial using a non-electroplating technique. For example, the coatingsof the present invention may be formed by immersing the substratematerial into a tin-silver bath maintained at a temperature of at least500° F. and preferably at a temperature in the range of from 500° F. toabout 900° F. The bath in a preferred embodiment comprises molten tinand silver and has a composition consisting of more than 1.0 wt % toabout 20 wt %, preferably from 2.0 wt % to 15 wt %, most preferably from3.0 wt % to 10 wt %, silver and the balance essentially tin. Thesubstrate material being immersed in the bath may be a continuous stripof metal or may be pieces of metal pre-stamped to form a particular typeof electrical or electronic connector. Alternatively, the coatedsubstrate material may be processed into a particular type of electricalor electronic connector after it has been removed from the bath. Forexample, the coated substrate material may be stamped into a particulartype of electrical or electronic connector after it has been removedfrom the bath.

Optionally, the coating bath may also contain an effective amount up toabout 5.0 wt %, preferably from 0.1 wt % to about 5.0 wt %, of at leastone hardening element selected from the group consisting of bismuth,silicon, copper, magnesium, iron, nickel, manganese, zinc, antimony, andmixtures thereof. For example, the bath may contain from 0.1 wt % to 1.5wt % copper.

As previously mentioned, the substrate material to be immersed in thebath may have any desired form such as a coil of substrate material. Thesubstrate material may be run through the bath either continuously ordiscontinuously using any suitable system known in the art. Further, ina preferred embodiment of the present invention, the substrate materialto be coated may be resident in the bath for a time period in the rangeof from 0.2 seconds to 10 seconds to form a coating having a thicknessin the range of from 0.00001″ to 0.001″.

It has been found by adding silver in the above specified amounts totin, one forms a coating with a hardened coating surface. This isdesirable because the harder the surface, the longer its cycle life. Aspreviously mentioned, pure tin coatings have a cycle life of 170 cyclesbefore the contact resistance rises above the standard of 10 milliohms.By adding 2.0 wt % silver to the tin, it has been found that the cyclelife can be increased to 250 cycles before the contact resistance risesabove the aforementioned standard. It has also been found that by adding5.0 wt % silver to the tin, the cycle life can be increased to 900cycles before the contact resistance rises above the aforementionedstandard.

It has also been found that by adding 5.0 wt % silver to the coating,the coating stays in a homogeneous state during the coating process. Inother words, the silver does not separate from the tin. This means thatafter the coating cools, the silver does not suspend or separate.

While the tin-silver coatings of the present invention are moreexpensive than pure tin coatings, they are substantially less in costthan gold coatings. Thus, the coatings of the present invention offersubstantial economic savings.

It has also been found that the life cycle of the coatings of thepresent invention may further be increased by applying a lubricant, suchas a synthetic hydrocarbon based grease, to the surfaces of thesubstrate material after formation of the tin-silver coating. It isexpected that coatings having as much as 10 wt % silver in the tincoating will have a life cycle of 1 million cycles when such a lubricantis used—a substantial increase over current life cycles.

The addition of silver in the above specified amounts to the coating hasother benefits. For example, the presence of silver increases thesurface melting point. Further, the presence of the silver helpseliminate fusing or arcing of the coated contacts or terminals. This isa highly desirable result because power sources for cars are increasingto a 42 volt system.

Coatings formed in accordance with the present invention are also freeof silver sulfamates and deleterious oxides.

Other advantages to the coatings of the present invention include theabsence of any inorganic materials in the coatings, the presence of astrong metallurgical bond between the coatings and the substratematerials, increased lubricity which enhances tool life, and thepresence of excellent electrical conductivity properties in excess of15.6% IACS, e.g. a 95% tin-5% silver coating has an electricalconductivity of 16.6% IACS and a 90% tin-10% silver coating has anelectrical conductivity greater than 20% IACS. Further, there is no needto add brighteners to the bath forming the coating.

In order to demonstrate the improvements offered by the coatings of thepresent invention, specimens were produced in accordance with the methodof the present invention which contained a substrate formed from acommercially available copper alloy designated as Alloy 4252 coated withpure tin, a substrate formed from the same material coated with 98 wt %tin and 2 wt % silver, a substrate formed from the same material coatedwith 95 wt % tin and 5 wt % silver, and a substrate formed from the samematerial coated with 90% tin and 10 wt % silver. The specimens were thentested to determine both the cycles before resistance increases to 10milliohms and the resistance value. The test showed that on the averagea pure tin coated substrate lasted 289 cycles before reaching 10milliohms and had a resistance value which ranged from 10.09 to 10.88.The test also showed that the 98-2 coating had an average cycle life of452 cycles and a resistance value that ranged from 10.16 to 10.74; thatthe 95-5 coating had an average cycle life of 399 cycles and aresistance value which ranged from 10.11 to 10.82; and that the 90-10coating had an average cycle life of 157 cycles and a resistance valuewhich ranged from 10.09 to 10.83. It was also found that as the silvercontent of the coating increased that the melting point of the coatingincreased. For example, a pure tin coating has a melting point of 231°C., the 95-5 tin-silver coating has a melting point in the range of 245°C. to 253° C., and the 90-10 tin-silver coating has a melting point of310° C.

Another test was conducted to determine the hardness of tin-silvercoatings in accordance with the present invention as compared to puretin coatings. The test was conducted using nanoidentation to determinewhether the indentation hardness of tin-based films was affected by theadditions of silver. To conduct the test specimens were prepared with apure tin coating on a copper alloy substrate, a 98 wt % tin-2 wt %silver coating on a copper alloy substrate, a 95 wt % tin-5 wt % silvercoating on a copper alloy substrate, and a 90 wt % tin-10 wt % silvercoating on a copper alloy substrate. Each of the tin based coatings wereapproximately 3000 nm thick. In order to eliminate interference from thesubstrate, the maximum indentation depth for the tin based coatings wasless than 33% of the coating thickness. At least three indentation testswere conducted on each sample with a tin based coating.

The results of the hardness test on the tin based films shows thathardness increases with increasing silver content in the coating. Theindentation hardness for the pure tin coating was 0.3 GPa, whereas thehardness for the 98-2 tin coating, the 95-5 tin coating and the 90-10tin coating were from 0.32 GPa to 0.41 Gpa. The FIGURE illustrates thehardness range for each of the tin-silver coatings. The test clearlyillustrates the improvements in hardness to be obtained by using atin-silver coating in accordance with the present invention.

While the coatings of the present invention have been described in thecontext of automotive applications, it should apparent to those skilledin the art that the coatings have utility in other electrical contact orelectrical terminal environments.

It is apparent that there has been provided a tin-silver coating whichfully satisfies the objects, means and advantages set forthhereinbefore. While the present invention has been described in thecontext of specific embodiments, other alternatives, modifications, andvariations will become apparent to those skilled in the art having readthe foregoing description. Accordingly, it is intended to embrace suchalternatives, modifications and variations as fall within the broadscope of the appended claims.

1. An automotive mating electrical connector comprising: an electricallyconductive material; and a non-electroplated coating formed on at leasta portion of the electrically conductive material, said coatingconsisting of 3.0 wt % to about 20 wt % silver and the balance tin, saidcoating having a melting point greater than 225° C., a thickness in therange of from 0.00001″ to 0.001″, and a nanoindentation hardness in therange of from 0.32 to 0.41 GPa.
 2. An automotive mating electricalconnector according to claim 1, wherein said silver content in saidcoating is in the range of from 3.0 wt % to 15 wt %.
 3. An automotivemating electrical connector according to claim 1, wherein said silvercontent in said coating is in the range of from 3.0 wt % to 10 wt %. 4.An automotive mating electrical connector comprising: an electricallyconductive material; and a non-electroplated coating formed on at leasta portion of the electrically conductive material, the coatingconsisting of more than 3.0 wt % to 20 wt % silver, at least oneaddition selected from the group consisting of silicon, magnesium, iron,manganese, zinc, and antimony in an amount effective to increase coatinghardness up to 5.0 wt %, and the balance tin, said coating having amelting point greater than 225° C., a thickness in the range of from0.00001″ to 0.001″, and having a nanoindentation hardness in the rangeof from 0.32 GPa to 0.41 GPa.
 5. An automotive mating electricalconnector according to claim 4, wherein said silver content of saidcoating is in the range of from 3.0 wt % to 15 wt %.
 6. An automotivemating electrical connector according to claim 4, wherein said silvercontent of said coating is in the range of from 3.0 wt % to 10 wt %. 7.An automotive mating electrical connector according to claim 4, whereinsaid at least one addition is present in an amount which does not causethe formation of deleterious oxides.
 8. An automotive mating electricalconnector according to claim 7, wherein said at least one addition ispresent in an amount ranging from 0.1 wt % to said amount which does notcause the formation of deleterious oxides.
 9. An automotive matingelectrical connector comprising: an electrically conductive substratematerial; and a non-electroplated layer of coating material on at leasta portion of said substrate material and said coating materialconsisting of 3.0 wt % to 20 wt % silver, copper in a range from 2.5 wt% to 5.0 wt %, and the balance tin, said coating material having amelting point greater than 225° C., a thickness in the range of from0.00001″ to 0.001″, and having a nanoindentation hardness in the rangeof from 0.32 to 0.41 GPa.
 10. An automotive mating electrical connectoraccording to claim 9, wherein said silver content of said coatingmaterial is in the range of from 3.0 wt % to 15 wt %.
 11. An automotivemating electrical connector according to claim 9, wherein said silvercontent of said coating material is in the range of from 3.0 wt % to 10wt %.
 12. An automotive mating electrical connector according to claim9, wherein said substrate material comprises a non-ferrous basedmaterial.
 13. An automotive mating electrical connector according toclaim 9, wherein said substrate material comprises a copper-telluriumalloy.
 14. An automotive mating electrical connector according to claim9, wherein said coating material directly contacts a surface of saidsubstrate material.
 15. An automotive mating electrical connectorcomprising: an electrically conductive substrate material; and anon-electroplated layer of coating material over at least a portion ofsaid substrate material, and said coating material consisting of 3.0 wt% to 20 wt % silver, at least one addition selected from the groupconsisting of bismuth, silicon, copper, magnesium, iron, nickel,manganese, zinc, and antimony in an amount effective to increase coatinghardness up to 5.0 wt %, and the balance tin, and said coating materialhaving a melting point greater than 225° C., a thickness in the range offrom 0.00001″ to 0.001″, and a nanoindentation hardness in the range offrom 0.32 GPa to 0.41 GPa.
 16. An automotive mating electrical connectoraccording to claim 15, wherein said silver is present in an amount from2.0 wt % to 15 wt %.
 17. An automotive mating electrical connectoraccording to claim 15, wherein said silver is present in an amount from3.0 wt % to 15 wt %.
 18. An automotive mating electrical connectoraccording to claim 15, wherein said substrate material is formed from anon-ferrous based material.
 19. An automotive mating electricalconnector according to claim 15, wherein said substrate material isformed from a copper-tellurium alloy.
 20. An automotive matingelectrical connector according to claim 15, wherein said at least oneaddition is present in an amount from 0.1 wt % up to an amount whichdoes not create deleterious oxides.
 21. An automotive mating electricalconnector to claim 15, wherein said coating material directly contacts asurface of said substrate material.
 22. A process for forming anautomotive mating electrical connector comprising the steps of:providing a substrate material to be coated; preparing a bath consistingof 3.0 wt % to 20 wt % silver and the balance tin; immersing saidsubstrate material in said bath to form a non-electroplated coatinglayer on said substrate material, which coating layer consists 3.0 wt %silver and the balance tin and which coating has a nanoindentationhardness in the range of 0.32 GPa to 0.41 GPa and a thickness in therange of from 0.00001″ to 0.001″; maintaining said bath at a temperaturegreater than 500° F. during said immersing step; and forming anautomotive mating electrical connector from the coated substratematerial.
 23. A process according to claim 22, wherein said preparingstep comprises preparing a bath consisting of from 3.0 wt % silver andthe balance tin.
 24. A process according to claim 22, wherein saidpreparing step comprises preparing a bath consisting of from 3.0 wt % to10 wt % silver and the balance tin.
 25. A process according to claim 22,wherein said maintaining step comprises maintaining said bath at atemperature of from 500° F. to 900° F. during said immersing step.
 26. Aprocess according to claim 22, wherein said immersing step comprisescontinuously passing said substrate material through said bath.
 27. Aprocess according to claim 22, wherein said immersing step comprisesdiscontinuously passing said substrate material through said bath.
 28. Aprocess according to claim 22, wherein said immersing step comprisesimmersing a batch of said substrate material into said bath andmaintaining said batch within said bath for a time period sufficient toform said coating.
 29. A process according to claim 22, furthercomprising keeping said substrate material resident in said bath for atime period in the range of 0.2 seconds to 10 seconds.
 30. A processaccording to claim 22, further comprising applying a lubricant tosurfaces of said substrate material after said immersing step.
 31. Aprocess for forming an automotive mating electrical connector comprisingthe steps of preparing a bath consisting of 3.0 wt % to 20 wt % silver,at least one addition selected from the group consisting of bismuth,silicon, magnesium, iron, manganese, zinc, and antimony in an amounteffective to increase coating hardness up to 5.0 wt %, and the balancetin; maintaining said bath at a temperature of at least 500° F.;immersing an electrically conductive substrate material in said bath fora resident time period of from 0.2 to 10 seconds to form a coating onthe substrate material; and forming an automotive mating electricalconnector from the coated substrate material.
 32. A process according toclaim 31, wherein said immersing step comprises continuously passingsaid substrate material through said bath.
 33. A process according toclaim 31, wherein said immersing step comprises discontinuously passingsaid substrate material through said bath.
 34. A process according toclaim 31, wherein said immersing step comprises introducing a batch ofsaid substrate material into said bath.
 35. A process according to claim31, wherein said maintaining step comprises maintaining said bath at atemperature in the range of 500° F. to 900° F.
 36. A process for formingan automotive mating electrical connector comprising the steps ofpreparing a bath consisting of 3.0 wt % to 20 wt % silver and thebalance tin; maintaining said bath at a temperature of at least 500° F.;immersing an electrically conductive substrate material in said bath fora resident time period of from 0.2 to 10 seconds; and forming anautomotive mating electrical connector from the coated substratematerial.
 37. A process according to claim 36, wherein said immersingstep comprises continuously passing said substrate material through saidbath.
 38. A process according to claim 36, wherein said maintaining stepcomprises maintaining said bath at a temperature in the range of 500° F.to 900° F.
 39. A process according to claim 36, further comprisingapplying a lubricant to surfaces of said substrate material after saidimmersing step.